docs: def _parse_request_method(request: web.Request):
          """Parse Access-Control-Request-Method header of the preflight request
          """
          method = request.headers.get(hdrs.ACCESS_CONTROL_REQUEST_METHOD)
          if method is None:
              raise web.HTTPForbidden(
                  text="CORS preflight request failed: "
                       "'Access-Control-Request-Method' header is not specified")
  
          # FIXME: validate method string (ABNF: method = token), if parsing
          # fails, raise HTTPForbidden.
  
          return method